Aqueous solutions of amines, such as diethanolamine or diisopropanolamine and sometimes containing physical solvents such as sulfolane, have been utilized to treat acid gas streams to remove unwanted components. This type of solvent absorption system is usually not "selective", in that CO.sub.2 is absorbed along with the sulfur-containing components, H.sub.2 S and COS, with correspondingly increased processing cost. Some aqueous amines, such as methyldiethanolamine (MDEA), with or without the addition of a physical solvent such as sulfolane, will selectively remove H.sub.2 S and yet permit CO.sub.2 to remain in the treated gas. The problem with this type of solvent absorption is that COS remains in the treated gas with the CO.sub.2. It is desirable and beneficial to arrive at a kinetic process for the selective removal of both COS and H.sub.2 S while permitting CO.sub.2 to remain in the gaseous stream.
Various patentees have attempted such processes but have not yet found a way to accomplish all of the desired process advantages in a relatively simple manner. In U.S. Pat. Nos. 4,524,050 and 4,482,529 (Chen et al), a process is disclosed for the catalytic hydrolysis of COS contained in acid gas streams. The patentees indicate that either a bicyclo tertiary amine ("bridgehead amine") or a bicyclo amidine can be utilized as a catalyst to hydrolyze COS to H.sub.2 S and CO.sub.2 and thereby eliminate COS as a noxious component from the gas stream. Other catalysts such as platinum on alumina, have been employed for the hydrolysis of COS, but such catalysts have problems of incomplete reaction, and they are limited by the equilibrium of the reaction if the product H.sub.2 S and CO.sub.2 are not immediately removed from the process environment. Also, monocyclic amines, such as 1,2-dimethylimidazole, have been utilized for the conversion of COS to H.sub.2 S and CO.sub.2. These disclosures, however, fail to combine the advantageous bicyclo amine or bicyclo amidine and a tertiary amine (not having substituents interconnected) to accomplish their respective absorptive and catalytic functions during acid gas treatment.
In U.S. Pat. No. 3,989,811 (Hill), a process is described for the treatment of sour gas streams containing significant quantities of H.sub.2 S, CO.sub.2 and COS, all of which are absorbed from the gas stream. Suitable solvents are disclosed as aqueous alkali metal carbonates and phosphate solutions, alkanol amines and sulfolanes. This disclosure lacks any appreciation of the use of a tertiary amine or a bridgehead amine for the purpose of selective removal of H.sub.2 S and COS without absorption of CO.sub.2. U.S. Pat. No. 4,397,660, Van der Pas-Toornstra, discloses a process for the removal of H.sub.2 S and CO.sub.2 from a gas mixture utilizing a tertiary amine and a physical absorbent such as Sulfinol. This is also a disclosure of a process which will eliminate CO.sub.2 from the acid gas stream in addition to H.sub.2 S and COS. As exemplary of how difficult it is to remove COS, without also removing CO.sub.2, U.S. Pat. No. 4,412,977, Fisch, discloses a process which desires to reduce the sulfur content of a gaseous mixture containing significant quantities of COS in addition to H.sub.2 S and CO.sub.2. As a result of this process, four separate gaseous streams are produced comprising a purified acid gas-free stream, a COS stream, an H.sub.2 S stream and a CO.sub.2 stream. Again, this process is selective to CO.sub.2 extraction in addition to the removal of H.sub.2 S and COS and fails to hydrolyze the COS to other components for their absorption in a tertiary amine.
A process for desulphurizing gases with an amine containing absorbent solution is disclosed in U.S. Pat. No. 4,532,116, Doerges et al. This process has as its objective the preparation of an extremely pure treated gas stream having no more than 3 ppm volume or more total sulphur compounds in the effluent gas. The particular amines chosen to perform this difficult task of removing COS in the presence of CO.sub.2 are secondary amines as are exemplified at Column 3 therein. In the instant process, secondary amines are neither desired nor required. The tertiary amines of this invention and the bridgehead amines are not disclosed or taught in this disclosure. In fact, all of the amines used by the patentees are very volatile to enhance condensation during downstream scrubbing. This requires a significant expenditure in energy and results in an economic loss to the process in order to prevent physical loss of the volatile amines. Another U.S. patent by Doerges et al, No. 4,504,499, describes a complex scheme utilizing similar secondary amines. The amines are selective for the removal of COS in the presence of CO.sub.2. However, it is necessary to regenerate the amines by a complex scheme, the object of which is to produce a pure CO.sub.2 stream, which can be discharged to the atmosphere without further treatment. Also, a rich H.sub.2 S stream is desired so that it may be fed directly to a Claus plant for production of sulphur. Another process utilizing secondary amines to absorb COS, and known to absorb nearly all of the CO.sub.2, is described in Archerd, U.S. Pat. No. 4,136,695.
In U.S. Pat. No. 3,965,244 (Sykes) a process is described for the selective removal of COS from a gas stream that contains H.sub.2 S and CO.sub.2. COS is selectively absorbed into a Sulfinol-D solution that is saturated with H.sub.2 S and CO.sub.2 and contains a high concentration of sulfolane to assist in COS absorption. The absorbed COS is hydrolyzed to H.sub.2 S and CO.sub.2, which are then returned to a gas phase. In the second step, H.sub.2 S is selectively removed from the CO.sub.2 -containing gas. Thus, a separate step is required to convert the COS to CO.sub.2 and H.sub.2 S. While triethanolamine (TEA) is disclosed in this patent, there is no recognition of the use of a tertiary amine with a bridgehead amine. Finally, in U.S. Pat. No. 2,726,992 (Easthagen et al), a process is described wherein diethanolamine is used for the removal of COS from a liquid petroleum gas. This type of absorption is non-selective and will absorb CO.sub.2 even though the same is not mentioned specifically in this disclosure.
A process heretofore has not been described utilizing a conjunct interaction of a bridgehead amine with a tertiary amine in the presence of a physical solvent selective for COS absorption. No hint or suggestion has been provided by these disclosures that the advantageous solvents of the different disclosures can be utilized to arrive at a process whereby COS and H.sub.2 S are selectively removed without absorbing CO.sub.2.